Dual voltage and current control feedback loop for an optical sensor system

ABSTRACT

A dual voltage and current control feedback control loop for an optical sensor system. A power supply provides a regulated DC voltage. A current source receives the regulated DC voltage and provides switched current to a light source. A current feedback representative of the current to the light source is provided to the power supply on a feedback path when the current source is driving the light source. A voltage feedback representative of the DC voltage is provided to the power supply on the feedback path when the current source is not driving the light source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from the following commonly owned U.S.Provisional Patent Applications: Ser. No. 61/165,171, Ser. No.61/165,181, Ser. No. 61/165,388, and Ser. No. 61/165,159, all of whichwere filed on Mar. 31, 2009.

This application is related to the following commonly-ownedapplications: U.S. Utility patent application Ser. No. 12/652,083,entitled “CURRENT SOURCE TO DRIVE A LIGHT SOURCE IN AN OPTICAL SENSORSYSTEM”; U.S. Utility patent application Ser. No. 12/652,089, entitled“OPTICAL SENSOR SYSTEM INCLUDING SERIES CONNECTED LIGHT EMITTINGDIODES”; and U.S. Utility patent application Ser. No. 12/652,095,entitled “HIGH VOLTAGE SUPPLY TO INCREASE RISE TIME OF CURRENT THROUGHLIGHT SOURCE IN AN OPTICAL SENSOR SYSTEM”; all filed on Jan. 5, 2010,and all of which are hereby incorporated by reference.

TECHNICAL FIELD

The present application relates to the sensors and, more particularly,to a dual voltage and current control feedback loop for an opticalsensor system.

BACKGROUND

Optical sensor systems may be used to locate and/or image an object bydetecting light reflected from the object. Such systems may include alight source that transmits light toward an object and a detector fordetecting portions of the transmitted light reflected by the object. Acharacteristic of the reflected light may be analyzed by the sensorsystem to determine the distance to an object and/or to generate anelectronic image of the object.

In one example, such a system may include a light source, such as one ormore light emitting diodes (LEDs), configured to transmit modulatedinfrared light (IR), i.e. IR light that is rapidly turned on and off.The detector may receive the reflected light and calculate the phaseshift imparted by reflection of the light back to the sensor. The timeof flight of the received light may be calculated from the phase shiftand distance to various points in the sensor field of view may becalculated by multiplying the time of flight and the velocity of thesignal in the transmission medium. By providing an array of receivingpixels in the detector, the distance signals associated with lightreceived at each pixel may be mapped to generate a three-dimensionalelectronic image of the field of view.

The manner of modulation of the light source in such systems is a factorin system performance. To achieve useful and accurate imaging, it isdesirable to modulate the light source at a high frequency, e.g. 40 MHz.In addition, it is desirable in such systems to modulate the lightsource with high efficiency and reliability, while maintainingreasonable cost of manufacture and a relatively small package size.

SUMMARY

In an embodiment, there is provided a light source circuit for anoptical sensor system. The light source circuit includes: a power supplyto provide a regulated direct current (DC) voltage output; a voltagefeedback circuit to provide a voltage feedback representative of the DCvoltage output on a feedback path to the power supply; a current sourcecoupled to the power supply to receive the regulated DC voltage outputand to provide a current output to a light source, the current sourcebeing configured to provide a current feedback representative of thecurrent output on the feedback path to the power supply; and a switch,whereby the current source is configured to provide the current outputto the light source and the power supply is configured to adjust the DCvoltage output in response to the current feedback when the switch isclosed and the power supply is configured to adjust the DC voltageoutput in response to the voltage feedback when the switch is open.

In a related embodiment, the current source may include an inductorconnected in series with a resistor; and a diode coupled in parallelwith the inductor and resistor; and wherein the current source isconfigured to provide the current output through the inductor to thelight source when the switch is closed and to divert current through theinductor to the diode when the switch is open. In a further relatedembodiment, the current source may include current monitor coupled tothe resistor and configured to provide the current feedback.

In another related embodiment, the voltage feedback circuit may includea voltage divider coupled to the DC voltage output. In yet anotherrelated embodiment, the light source may include a plurality of seriesconnected light emitting diodes. In still another related embodiment,the circuit may further include a drive circuit to open and close theswitch at a predetermined frequency. In a further related embodiment,the predetermined frequency may be about 40 MHz.

In another embodiment, there is provided an optical sensor system. Theoptical sensor system includes a controller; a light source circuitcoupled to the controller to drive a light source in response to controlsignals from the controller, the light source circuit comprising: apower supply to provide a regulated direct current (DC) voltage output;a voltage feedback circuit to provide a voltage feedback representativeof the DC voltage output on a feedback path to the power supply; acurrent source coupled to the power supply to receive the regulated DCvoltage output and to provide a current output to the light source, thecurrent source being configured to provide a current feedbackrepresentative of the current output on the feedback path to the powersupply; and a switch, whereby the current source is configured toprovide the current output to the light source and the power supply isconfigured to adjust the DC voltage output in response to the currentfeedback when the switch is closed and the power supply is configured toadjust the DC voltage output in response to the voltage feedback whenthe switch is open; transmission optics to direct light from the lightsource toward an object; receiver optics to receive light reflected fromthe object; and detector circuits to convert the reflected light to oneor more electrical signals, the controller being configured to provide adata signal output representative of a distance to at least one point onthe object in response to the one or more electrical signals.

In a related embodiment, the current source may include an inductorconnected in series with a resistor; and a diode coupled in parallelwith the inductor and resistor; and wherein the current source isconfigured to provide the current output through the inductor to thelight source when the switch is closed and divert current through theinductor to the diode when the switch is open. In a further relatedembodiment, the current source may include a current monitor coupled tothe resistor and configured to provide the current feedback.

In another related embodiment, the voltage feedback circuit may includea voltage divider coupled to the DC voltage output. In yet anotherrelated embodiment, the light source may include a plurality of seriesconnected light emitting diodes. In still yet another relatedembodiment, the system may further include a drive circuit to open andclose the switch at a predetermined frequency. In a further relatedembodiment, the predetermined frequency may be about 40 MHz.

In another embodiment, there is provided a method of providing currentto a light source under the control of a switch in an optical sensorsystem. The method includes providing the current through a currentsource to the light source when the switch is closed; adjusting a DCinput voltage to the current source in response to a current feedbacksignal provided on a feedback path when the switch is closed, thecurrent feedback signal being representative of current provided to thelight source; and adjusting the DC input voltage to the current sourcein response to a voltage feedback signal provided on the feedback pathwhen the switch is open, the voltage feedback signal beingrepresentative of the DC input voltage.

In a related embodiment, providing may include providing the currentthrough a current source to the light source when the switch is closed,wherein the current source comprises an inductor connected in serieswith a resistor, a diode coupled in parallel with the inductor andresistor, and a current monitor coupled to the resistor and configuredto provide the current feedback signal. In another related embodiment,the method may further include opening and closing the switch at apredetermined frequency. In a further related embodiment, opening andclosing may include opening and closing the switch at a predeterminedfrequency that is about 40 MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages disclosedherein will be apparent from the following description of particularembodiments disclosed herein, as illustrated in the accompanyingdrawings in which like reference characters refer to the same partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating the principlesdisclosed herein.

FIG. 1 is a block diagram of an optical sensor system according toembodiments described herein.

FIG. 2 is a block diagram of optical sensor system light source circuitsaccording to embodiments described herein.

FIG. 3 is a block diagram of optical sensor system light source circuitsincluding a dual voltage and current control feedback loop according toembodiments described herein.

FIG. 4 is a block diagram of optical sensor system light source circuitsincluding a dual voltage and current control feedback loop to drive alight source including a plurality of series connected LEDs according toembodiments described herein.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of an optical sensor system 100according to embodiments disclosed herein. In general, the opticalsensor system 100 emits light 102, e.g. infrared (IR) light, that isreflected by an object 104, and receives the reflected light 106 toidentify the distance to the object 104 and/or to map an image of theobject 104. In some embodiments, for example, the system may beimplemented as a collision avoidance sensor, e.g. a back-up sensor, foran automotive vehicle. In a back-up sensor application, for example, thesystem provides a data output 108 indicating distance from the rear ofthe vehicle to an object 104 for assisting a driver of the vehicle inavoiding inadvertent contact with the object 104 when moving in reverse.Although systems and methods consistent with the present disclosure maybe described in connection with a particular application, those ofordinary skill in the art will recognize that a wide variety ofapplications are possible. For example, systems and methods consistentwith the present disclosure may be implemented in optical sensors forrange finding applications, or any application involving identificationand/or imaging of a target object.

Those of ordinary skilled in the art will recognize that the opticalsensor system 100 has been depicted in highly simplified form for easeof explanation. The optical sensor system 100 shown in FIG. 1 includescontroller/processing circuits 110, light source circuits 112,transmission optics 114, receiver optics 116 and detector circuits 118.The controller/processing circuits 110 may be known circuits forcontrolling modulation of a light source of the light source circuitsand for processing received data to generate an output data streamrepresentative of the distance from the sensor to the object and/or anelectronic image of the object. Controller/processing circuits 110 may,for example, be any of the depth sensor controller/processing circuitscommercially available from Canesta, Inc. of Sunnyvale, Calif.

The light source circuits 112 may include known circuitry for drivingthe light source in response to control outputs from thecontroller/processing circuits 110, and may include circuitry consistentwith the present disclosure. The transmission optics 114 may includeknown optical components for directing light output from the lightsource to provide a system field of view encompassing the object(s) ofinterest. The receiver optics 116 may include known optical componentsfor receiving light reflected from the object of interest and directingthe received light to the detector circuits 118. The detector circuits118 may include known light detectors, e.g. arranged in an array ofpixels, for converting the received light into electrical signalsprovided to the control/processing circuits 110. The detector circuits118 may, for example, be any of the detector circuits commerciallyavailable from Canesta, Inc. of Sunnyvale, Calif. The control processingcircuits 110 may calculate distance to various points on the object andwithin the system field of view, e.g. using phase shift in the receivedlight to calculate time of flight and distance, to provide the dataoutput indicating distance to the object and/or mapping the object toprovide a three-dimensional image thereof.

FIG. 2 is a simplified block diagram of the light source circuits 112according to embodiments described herein. The light source circuits 112include a power supply 202, a current source 204 coupled to the outputof the power supply 202, one or more light sources 206 coupled to thecurrent source 204, an optional high voltage supply 208 coupled to thecurrent source 204, and driver circuits 210 for controlling switches S1and S2 to turn the one or more light sources 206 off and on at apredetermined frequency, i.e. modulate the one or more light sources206. The term “coupled” as used herein refers to any connection,coupling, link or the like by which signals carried by one systemelement are imparted to the “coupled” element. Such “coupled” devices,or signals and devices, are not necessarily directly connected to oneanother and may be separated by intermediate components or devices thatmay manipulate or modify such signals. The driver circuits 210 may takeone of any known configuration or configuration described herein.

The power supply 202 may take any known configuration for receiving aninput voltage from an input voltage source 212 and providing a regulateddirect current (DC) voltage output. The input voltage source 212 may be,for example as shown in FIG. 2, a DC source, e.g. a vehicle battery, andthe power supply 202 may be, for example as shown in FIG. 2, a knownDC-DC converter for converting the DC source voltage to a regulated DCvoltage at the output of the power supply 202. Known DC-DC convertersinclude, for example, buck converters, boost converters, single endedprimary inductor converter (SEPIC), etc. In some embodiments, a SEPICconverter may be used to allow a regulated DC output voltage that isgreater than, less than, or equal to the input voltage. SEPIC converterand SEPIC converter controller configurations are well-known to those ofordinary skill in the art. One SEPIC converter controller useful inconnection a system consistent with the present disclosure iscommercially available from Linear Technology Corporation, as modelnumber LTC1871®. Although FIG. 2 shows a DC source voltage, those ofordinary skill in the art will recognize that an alternating current(AC) input may be used, and the power supply 202 may then include aknown AC-DC converter for providing a regulated DC output voltage.

The current source 204 may provide a constant current to the one or morelight sources 206 for energizing the one or more light sources 206 whenthe switch S1 is closed by the driver circuits 210. The switch S1 isillustrated in diagrammatic form for ease of explanation, but may takethe form of any of a variety of configurations known to those ofordinary skill in the art. For example, the switch S1 may be atransistor configuration that conducts current under the control of thedriver circuit output.

The driver circuits 210 may be configured to open and close the switchS1 at a predetermined frequency under the control of control signals 214from the controller/processing circuits 110. In some embodiments, forexample, the driver circuits 210 may open and close the switch S1 at afrequency of about 40 MHz. The current source 204 may thus provide adriving current to the one or more light sources 206 at thepredetermined frequency for modulating the one or more light sources206, i.e. turning the one or more light sources 206 on and off.

The optional high voltage supply 208 may be coupled to the one or morelight sources 206 through the switch S2. The switch S2 may be closed bythe driver circuits 210 under the control of control signals from thecontroller/processing circuits 110 during the start of the “on” time forthe one or more light sources 206. The optional high voltage supply 208may thus increase the voltage across the one or more light sources 206to a voltage higher than can be established by the current source 204 todecrease the rise time of the current through the one or more lightsources 206. After the start of the “on” time for the one or more lightsources 206, the switch S2 may open to disconnect the optional highvoltage supply 208 from the one or more light sources 206, and thecurrent source 204 may drive the one or more light sources 206 throughthe rest of the “on” time.

The switch S2 is illustrated in diagrammatic form for ease ofexplanation, but may take the any of a variety of configurations knownto those of ordinary skill in the art. For example, the switch S2 may bea transistor configuration that conducts current under the control of anoutput of the driver circuits 210. In addition, the switch S2 may beincorporated into the optional high voltage supply 208 or be separatetherefrom.

According to embodiments described herein, the current source 204 mayprovide current feedback V_(C) to the power supply 202. The currentfeedback V_(C) is representative of the current provided by the currentsource 204 to the one or more light sources 206 when the switch S1 isclosed by the driver circuits 210. In addition, a voltage feedbackcircuit may provide a voltage feedback V_(F) to the power supplyrepresentative of the regulated output voltage Vs of the power supply202. A variety of voltage feedback circuit configurations will be knownto those of ordinary skill in the art. In FIG. 3, the voltage feedbackcircuit is a voltage divider between a voltage V_(s) and groundestablished by series connection of a resistor R2 and a resistor R3. Thevalues of the resistor R2 and the resistor R3 may be selected in a knownmanner to scale the voltage feedback V_(F) to a range that meets therequirements of the power supply 202. It is to be understood, however,that any voltage feedback circuit known to those of ordinary skill inthe art may be implemented.

The voltage feedback V_(F) and current feedback V_(C) may be coupled topower supply 202 on a feedback path 216, which in some embodiments isthe same feedback path and in other embodiments may be a differentfeedback path. In this configuration, the power supply 202 may beconfigured to adjust its output voltage V_(s) (also referred tothroughout as supply voltage V_(s)) in response to the higher of thecurrent feedback V_(C) and the voltage feedback V_(F). For example, whenthe switch S1 is closed, the current feedback V_(C) may be greater thanthe voltage feedback V_(F) and the power supply 202 may be configured toadjust the supply voltage V_(s) in response to the current feedbackV_(C) to a voltage that will maintain a constant current from thecurrent source to the light source(s). When the switch S1 is open, thevoltage feedback V_(F) may be greater than current feedback V_(C) andthe power supply 202 may be configured to adjust the supply voltageV_(s) in response to the voltage feedback V_(F) to maintain a constantsupply voltage V_(s).

A variety of configurations for providing an adjustable supply voltagein response to the feedback on the feedback path 216 are well-known tothose of ordinary skill the art. In one embodiment, the power supply 202may be configured as a known converter, e.g. a SEPIC converter, and aknown converter controller, e.g. a SEPIC controller configured tocontrol the converter voltage output in response to the feedback. Forexample, the voltage feedback V_(F) and the current feedback V_(C) maybe coupled on the feedback path 216 to the F_(B) input of an LTC1871®SEPIC converter controller available from Linear Technology Corporation(not shown), which is configured to regulate the output voltage of aSEPIC converter based on an internal reference.

FIG. 3 illustrates a block diagram of optical sensor system light sourcecircuits including a dual voltage and current control loop according toembodiments described herein. In FIG. 3, the current source 204 aincludes a resistor R1 in series with an inductor L1, and a diode D1coupled in parallel across the series combination of the resistor R1 andthe inductor L1.

As shown, the regulated DC output V_(s) of the power supply 202 may becoupled to the input of the current source 204 a at the resistor R1. Thedriver circuits 210 may open and close the switch S1 at a highfrequency, e.g. 40 MHz. When the switch S1 is closed, a current I_(s)flows through the series combination of the resistor R1 and the inductorL1 and to the one or more light sources 206 for energizing the one ormore light sources 206. The inductor L1 thus establishes a constantcurrent source and limits the current I_(s) through the one or morelight sources 206 when the switch S1 is closed. When the switch S1 isopen, however, no current flows through the one or more light sources206, and a current I_(L) through the inductor L1 is diverted through thediode D1 to maintain current through the inductor L1.

As shown, a current monitor 304 may be coupled across the resistor R1for sensing the voltage drop across the resistor R1 and providing thecurrent feedback voltage output V_(C) representative of the currentthrough the resistor R1. The current monitor 304 may take anyconfiguration known to those of ordinary skill in the art. In someembodiments, for example, the current monitor 304 may be configuredusing a current shunt monitor available from Texas Instruments® undermodel number INA138. The current monitor 304 may provide the currentfeedback output voltage V_(C) to the power supply 202, e.g. through thediode D2. In some embodiments, the diode D2 may be provided in a knownideal diode configuration to minimize the voltage drop across the diode.

The current feedback V_(C) and voltage feedback V_(F) may be coupled tothe power supply 202 on the common path 216. In response to the feedbackfrom the current monitor 304 and during the time when the switch S1 isclosed, the current feedback V_(C) may be greater than the voltagefeedback V_(F). The diode D2 may then conduct and the power supply 202may be configured to adjust the supply voltage V_(s) in response to thecurrent feedback V_(C) to a voltage that will allow the inductor L1 torecharge. When the switch S1 is open, the voltage feedback V_(F) may begreater than the current feedback V_(C) and the diode D2 may be in anon-conducting state. The power supply 202 may then adjust the supplyvoltage V_(s) in response to the voltage feedback V_(F) to maintain aconstant supply voltage V_(s).

A constant current may thus be established through the inductor L1 whenthe switch S1 is closed, i.e. when the one or more light sources is/are“on” and emitting light. As shown in FIG. 4, a current source 204 aconsistent with the present disclosure may be implemented in a systemwherein a light source 206 a includes a plurality of infraredlight-emitting diodes (LEDs) D3, D4, D5, and D6 connected in series.Although, as shown in FIG. 4, there are four series connected LEDs D3,D4, D5, and D6, it is to be understood that any number of LEDs may beconnected in series to provide a light source consistent with thepresent disclosure. As shown in FIG. 4, driving current from the currentsource 204 a is provided to the plurality of infrared LEDs D3, D5, D5,and D6 through a diode D7, and diodes D8, D9, D10, D11 are coupledacross the plurality of infrared LEDs D3, D4, D5, and D6, respectively,to take up any back voltage across the series connected plurality ofinfrared LEDs D3, D4, D5, and D6. The current source 204 a may thusprovide constant current through the series connected plurality ofinfrared LEDs D3, D4, D5, and D6 to allow switching/modulation of theLED output at relatively high frequency, e.g. 40 MHz. Connecting theplurality of infrared LEDs D3, D4, D5, and D6 in series avoids phasedifferences between LED outputs and provides cost efficiency.

Unless otherwise stated, use of the word “substantially” may beconstrued to include a precise relationship, condition, arrangement,orientation, and/or other characteristic, and deviations thereof asunderstood by one of ordinary skill in the art, to the extent that suchdeviations do not materially affect the disclosed methods and systems.

Throughout the entirety of the present disclosure, use of the articles“a” or “an” to modify a noun may be understood to be used forconvenience and to include one, or more than one, of the modified noun,unless otherwise specifically stated.

Elements, components, modules, and/or parts thereof that are describedand/or otherwise portrayed through the figures to communicate with, beassociated with, and/or be based on, something else, may be understoodto so communicate, be associated with, and or be based on in a directand/or indirect manner, unless otherwise stipulated herein.

Although the methods and systems have been described relative to aspecific embodiment thereof, they are not so limited. Obviously manymodifications and variations may become apparent in light of the aboveteachings. Many additional changes in the details, materials, andarrangement of parts, herein described and illustrated, may be made bythose skilled in the art.

What is claimed is:
 1. A light source circuit for an optical sensorsystem, the circuit comprising: a power supply to provide a regulateddirect current (DC) voltage output to a light source; a voltage feedbackcircuit to provide a voltage feedback directly from the DC voltageoutput of the power supply prior to the light source wherein the voltagefeedback circuit provides the voltage feedback on a feedback path to thepower supply with a voltage divider between the DC voltage output andground; a current source coupled to the power supply to receive theregulated DC voltage output and to provide a current output to a lightsource, the current source being configured to provide a currentfeedback representative of the current output on the feedback path tothe power supply prior to the light source; and a switch between thelight source and ground, whereby the current source is configured toprovide the current output to the light source and the power supply isconfigured to adjust the DC voltage output in response to the currentfeedback when the switch is closed and the power supply is configured toadjust the DC voltage output in response to the voltage feedback whenthe switch is open.
 2. The light source circuit according to claim 1,wherein the current source comprises: an inductor connected in serieswith a resistor; and a diode coupled in parallel with the inductor andresistor; and wherein the current source is configured to provide thecurrent output through the inductor to the light source when the switchis closed and to divert current through the inductor to the diode whenthe switch is open.
 3. The light source circuit according to claim 2,wherein the current source comprises a current monitor coupled to theresistor and configured to provide the current feedback.
 4. The lightsource circuit according to claim 1, wherein the voltage feedbackcircuit comprises a voltage divider coupled to the DC voltage output. 5.The light source circuit according to claim 1, wherein the light sourcecomprises a plurality of series connected light emitting diodes.
 6. Thelight source circuit according to claim 1, further comprising a drivecircuit to open and close the switch at a predetermined frequency. 7.The light source circuit according to claim 6, wherein the predeterminedfrequency is about 40 MHz.
 8. An optical sensor system comprising: acontroller; a light source circuit coupled to the controller to drive alight source in response to control signals from the controller, thelight source circuit comprising: a power supply to provide a regulateddirect current (DC) voltage output; a voltage feedback circuit toprovide a voltage feedback directly from the DC voltage output of thepower supply prior to the light source wherein the voltage feedbackcircuit provides the voltage feedback on a feedback path to the powersupply with the DC voltage output and ground established by seriesconnection of first resistor and second resistor and a voltage feedbackand ground established by a connection of the second resistor, a currentsource coupled to the power supply to receive the regulated DC voltageoutput and to provide a current output to the light source, the currentsource being configured to provide a current feedback directly from thecurrent output of the current source prior to the light source whereinthe current feedback provides the current feedback on the feedback pathto the power supply, and a switch subsequent the light source, wherebythe current source is configured to provide the current output to thelight source and the power supply is configured to adjust the DC voltageoutput in response to the current feedback when the switch is closed andthe power supply is configured to adjust the DC voltage output inresponse to the voltage feedback when the switch is open; transmissionoptics to direct light from the light source toward an object; receiveroptics to receive light reflected from the object; and detector circuitsto convert the reflected light to one or more electrical signals, thecontroller being configured to provide a data signal outputrepresentative of a distance to at least one point on the object inresponse to the one or more electrical signals.
 9. The optical sensorsystem according to claim 8, wherein the current source comprises: aninductor connected in series with a resistor; and a diode coupled inparallel with the inductor and resistor; and wherein the current sourceis configured to provide the current output through the inductor to thelight source when the switch is closed and divert current through theinductor to the diode when the switch is open.
 10. The optical sensorsystem according to claim 9, wherein the current source comprises acurrent monitor coupled to the resistor and configured to provide thecurrent feedback.
 11. The optical sensor system according to claim 8,wherein the voltage feedback circuit comprises a voltage divider coupledto the DC voltage output.
 12. The optical sensor system according toclaim 8, wherein the light source comprises a plurality of seriesconnected light emitting diodes.
 13. The optical sensor system accordingto claim 8, further comprising a drive circuit to open and close theswitch at a predetermined frequency.
 14. The optical sensor systemaccording to claim 13, wherein the predetermined frequency is about 40MHz.
 15. A method of providing current to a light source under thecontrol of a switch in an optical sensor system, the method comprising:providing the current through a current source to the light source whenthe switch is closed wherein the switch couples the light source toground; adjusting a DC input voltage to the current source in responseto a current feedback signal provided on a feedback path when the switchis closed, the current feedback signal measured from current providedprior to the light source; and adjusting the DC input voltage to thecurrent source in response to a voltage feedback signal provided on thefeedback path when the switch is open, the voltage feedback signal beingmeasured prior to the light source from the DC input voltage using avoltage divider between the input voltage and ground.
 16. The methodaccording to claim 15, wherein providing comprises: providing thecurrent through a current source to the light source when the switch isclosed, wherein the current source comprises an inductor connected inseries with a resistor, a diode coupled in parallel with the inductorand resistor, and a current monitor coupled to the resistor andconfigured to provide the current feedback signal.
 17. The methodaccording to claim 15, further comprising opening and closing the switchat a predetermined frequency.
 18. The method according to claim 17,wherein opening and closing comprises opening and closing the switch ata predetermined frequency that is about 40 MHz.
 19. The light sourcecircuit according to claim 1, wherein the voltage feedback circuit toprovide the voltage feedback representative of the DC voltage output onthe feedback path to the power supply with the voltage divider betweenthe DC voltage output and ground is established by series connection ofa first resistor and a second resistor.
 20. The light source circuitaccording to claim 19, wherein the voltage feedback and the ground isestablished by a connection of the second resistor.